The formation process can be regarded as an evolution chain from the mass failure on slope, to initial debris flows formation through soil-water interaction in gully, and then to eventual debris flow via influx processes in watershed. It is different from the initiation process on a single slope due to the influx processes were involved, which result in the difference of the activities and rainfall thresholds.  

Based on the characteristics aforementioned, Dr. Guo Xiaojun of Institute of Mountain Hazards and Environment, Chinese Academy of Sciences (IMHE, CAS), and his team analyzed the material supplying, triggering rainfall conditions and activities of debris flows in watershed, and then illustrated the debris flow formation and development processes and mechanism, based on a typical case. The debris flow was supplied by the widely distributed failures dominated byrill erosions. The intermittent supplying increased the size and duration of debris flow. While the landslide dam failures provided most amounts for debris flows, and amplified the discharge suddenly. During these processes,the debris flow velocity and density increased as well. The study showed that rainfall thresholds required to trigger debris flows in large watersheds were considerably higher than those on in small watersheds and/or on small slopes in lower-order valleys. The latter are more easily to be triggered as I = 4.4D^-0.70 (2h <D <37h）and the former as I = 14.7D^-0.79 (2h <D <56h). The debris flow frequency followed an exponentially decreasing tendency with watershed area as F = 2.04exp(-0.048A), and the average magnitudes linearly increased with watershed area as M = 9.66A-6.8. 

The research improved the knowledge of debris flow formation processes in watershed, and moreover, the methods of rainfall thresholds identification and mass amount calculation were detailed illustrated, which helped for the promotion of field investigation of debris flow.  

This research was supported by both the National Natural Science Foundation of China (Grant 41301008) and the Chinese Academy of Science (Grant No. KZZD-EW-05) 

The research achievements have been published in Landslides, 13(1): 25-37. 

 Full text URL: http://link.springer.com/article/10.1007%2Fs10346-014-0541-6